Mastocytosis
Mastocytosis (also referred to as mast cell disease) is defined as a clonal, neoplastic proliferation and accumulation of mast cells in one or multiple organs. Clinical signs and symptoms result from the release of chemical mediators and by infiltration of tissues (e.g., bone marrow, spleen, lymph nodes, liver, and gastrointestinal tract) by neoplastic mast cells. Mast cells are bone marrow derived cells that produce histamine and other substances causing allergic and anaphylactic reactions. Accumulation of mast cells in body organs can inhibit the functionality of the organ and eventually cause degeneration. Mastocytosis usually involves the skin and bone marrow, but may also involve other internal organs.
Diagnosis and Classification of Mastocytosis
Clinical advances have cumulated in development of the World Health Organization (WHO) consensus classification system for mastocytosis (Table 1) (Horny H P et al., in World Health Organization Classification of Tumours. Pathology and Genetics of Tumours of Haematopoietic and Lymphoid Tissues. Lyon, France: IARC Press; 2008. pp 54-63; or Arock, et al. Eur J Haematol. 2015 June; 94(6):474-90). Based upon clinical findings and symptoms, seven major categories of mastocytosis patients have been identified: cutaneous mastocytosis (CM) and six main variants of systemic mastocytosis (SM): indolent SM, SM with associated clonal hematological nonmast-cell lineage disease (SM-AHNMD), aggressive SM, mast cell leukemia, mast cell sarcoma, and extracutaneous mastocytoma. Prognosis relates to the SM variant and extends from a normal life expectancy in CM or indolent SM, to only a few months in mast cell leukemia (Valent P, et al., Br J Haematol 2003; 122:695-717).
A further possible distinction of mastocytosis based on WHO consensus classification system is as follows:                indolent forms of mastocytosis which are selected from smoldering systemic (SSM), indolent systemic (ISM) and cutaneous mastocytosis (CM), each being as defined in the WHO consensus classification system for mastocytosis; and        aggressive forms of mastocytosis which are selected from aggressive systemic mastocytosis (ASM), systemic mastocytosis associated with another clonal hematological non-mast cell lineage disease (SM-AHNMD), mast cell leukemia (MCL), mast cell sarcoma (MCS), and extracutaneous mastocytoma, each being as defined in the WHO consensus classification system for mastocytosis.        
TABLE 1Official WHO classification (Horny HP et al., in World Health OrganizationClassification of Tumours. Pathology and Genetics of Tumours of Haematopoietic andLymphoid Tissues. Lyon, France: IARC Press; 2008).AbbreviationsWHO termsDiagnosticCMCutaneous MastocytosisTypical skin lesions: eithermaculopapular, urticaria pigmentosa,mastocytoma.Typical infiltrate of mast cell in skin(no other tissue involvement)ISMIndolent Systemic MastocytosisMast cell infiltration in at least 1extracutaneous tissue.No B and C FindingsSSMSmoldering Systemic MastocytosisMast cells in bone marrow >5%.At least two B-Findings.No C-FindingSM-AHNMDSystemic Mastocytosis with anAssociated with myelodysplasia andAssociated clonal Hematologicmyeloproliferative syndrome andNon Mast cell lineage Diseasesometimes with an acute leukemia orlymphomaASMAggressive Systemic MastocytosisAt least one C-FindingMCLMast Cell LeukemiaLarge numbers of atypical mast cellsin the peripheral blood.Mast cells in bone marrow smears(≥20%).MCSMast Cell SarcomaExtracutaneous Mastocytoma
In most of these categories (CM, ISM, SM-AHNMD, ASM, MCL), subvariants have been identified, based on clinical and/or biological features [for example, see Table 1 of Arock, et al. Eur J Haematol. 2015 June; 94(6):474-90]. WHO has also provided diagnostic criteria, with further categorization according to the presence of B-(‘Borderline Benign’) findings and C-(‘Consider Cytoreduction’) findings, which reflect the disease burden (B) and disease aggressiveness (C), respectively [for example, see Tables 2 and 3 of Arock, et al. Eur J Haematol. 2015 June; 94(6):474-90]. For example, patients who have one or more C-findings are categorized as ASM or MCL and are candidates for therapeutic cytoreduction. Variants on the WHO diagnostic criteria are also used as appropriate.
The WHO diagnostic criterion for SM requires confirmation of one major and one minor criterion, or three minor criteria from a list of specific diagnostic findings (Table 2). The major criterion requires identification of multifocal dense infiltrates of mast cells in the marrow or other extracutaneous organ; minor criteria include: (1) spindle shaped or atypical morphology of mast cells, (2) detection of the D816V c-Kit mutation, (3) mast cell expression of CD2 and/or CD25 in addition to normal mast cell markers (e.g., tryptase and CD117), and (4) a serum tryptase level >20 ng/ml in the absence of another myeloid disorder. More indolent forms of SM are characterized by “B” findings (e.g., organ involvement without dysfunction) and can be distinguished from aggressive subtypes categorized by “C” or clinical findings associated with organ dysfunction. Cytoreductive therapies are usually reserved for patients with “C” findings, or for patients with mediator symptoms causing substantial morbidity and refractory to standard medications such as antihistamines, leukotriene antagonists, and mast cell stabilizers.
TABLE 2Biological and Clinical Findings as per WHO definition (Horny HP et al., inWorld Health Organization Classification of Tumours. Pathology and Genetics ofTumours of Haematopoietic and Lymphoid Tissues. Lyon, France: IARC Press; 2008).B findingsC findingsHigh Mast Cell burden:OrganopathiesInfiltration grade in bone marrow (bm) >Bone Marrow: cytopenia30% by histology and/plus serumANC <1 000/μLtryptase >200 ng/ml.Hb <10 g/dlDysmyelopoiesis:Plt <100 00/μLHypercellular marrow with loss of fat(one or more found).cells or discrete signs of MyelodysplasiaLiver: palpable Hepatomegaly withor Myeloproliferation, normal bloodascites, abnormal liver function testscounts or slight persisting deviationand/or portal hypertension.without progression.Spleen: palpable splenomegalyOrganomegaly:with hypersplenism.Palpable Hepatomegaly without ascitesGI tract: malabsorption withor other signs of organ impairmenthypoalbuminemia and weight loss.or/and Lymphadenopathy palpable orSkeleton: bone lesions with large-visceral LN-enlargement found in US orsized osteolyses or/and severeCT (>2 cm) and/or Palpable Splenomegalyosteoporosis with consecutive pathologic without hypersplenism.fractures.
CM is characterized by the presence of skin lesions in the absence of bone marrow or other internal organ infiltration by mast cells. In contrast to systemic mastocytosis, there are no well-defined pathologic criteria for diagnosis of CM. Diagnosis is generally established by observation of typical lesions of urticaria pigmentosa or mastocytoma, and by skin biopsies showing increased numbers of mast cells in the absence of other inflammatory cells, particularly in the upper dermis around blood vessels.
The aggressive forms of mastocytosis are rare (<10% of all cases) and require specific treatment aimed at reducing mast cell infiltration and activity. In the vast majority of cases (>90%), mastocytosis presents as an indolent form of the disease, e.g., smoldering SM, indolent SM or CM.
Role of Mast Cells in Inflammation
Mast cells are characterized by their heterogeneity, not only regarding tissue location and structure but also at functional and histochemical levels. Mast cell activation is followed by the controlled release of a variety of mediators that are essential for the defense of the organism against invading pathogens. By contrast, in the case of hyperactivation of mast cells, uncontrolled hypersecretion of these mediators is deleterious for the body. Mast cells produce a large variety of mediators categorized here into three groups:                Preformed granule-associated mediators (histamines, proteoglycans, and neutral proteases);        Lipid-derived mediators (prostaglandins, thromboxanes and leucotrienes);        Various cytokines (including the interleukins: IL-1, IL-2, IL-3, IL-4, IL-5, IL-6, IL-8 and tumor necrosis factor alpha TNF-α, GM-CSF, MIP-1α, MIP-1β and IFN-γ).        
Human mast cells constitutively express a number of receptors for different biological molecules. Among these receptors, whose ligation induces the activation of mast cells, the best known is the high affinity receptor for IgE (FccRI). Binding of IgE-multivalent antigen complexes to FccRI leads to receptor aggregation and internalization, signaling, and degranulation. This can be accompanied by the transcription of cytokine genes, thus, perpetuating the inflammatory response. Moreover, triggering of mast cells leads to the secretion of diverse pre-formed and/or de novo synthesized mediators, such as vasoactive amines (histamine, serotonin), sulfated proteoglycans, lipid mediators (prostaglandin D2, leucotrienes), growth factors, proteases, cytokines and chemokines as described previously. These mediators can, alone or in synergy with macrophage-derived and T cell-derived cytokines, generate a complex inflammatory response and induce the recruitment and activation of inflammatory cells to the site of degranulation.
Treatment of Mastocytosis
The treatment of mastocytosis, and in particular the long-term management of indolent forms of mastocytosis, remains a challenge to clinicians because of the diversity and complexity of the disease itself and the lack of a standard and highly effective therapy. None of these approved drugs represent a cure for the disease, no therapy available effectively destroys the mast cells responsible for mastocytosis; moreover, their efficacy is limited and may decrease over time, with undesirable side effects reported. In general, management of patients within all categories of mastocytosis includes: (i) avoidance of factors triggering acute mediator release, (ii) symptomatic treatment of acute mast cell mediator release, (iii) treatment of chronic mast cell mediator release, and if indicated (iv) an attempt to treat organ infiltration by mast cells. However, even with the help of appropriate symptomatic treatments, indolent forms of mastocytosis can have a profoundly negative impact on quality of life, with many of the symptoms and their associated disabilities often being unrecognized as manifestations of mastocytosis for several years.
In a recent retrospectively studied of Mayo Clinic patients who met the 2008 WHO diagnostic criteria for SM and had received at least one of four major cytoreductive drugs including: interferon-alpha with or without prednisone (IFN-α), hydroxyurea (HU), imatinib mesilate (IM) or Cladribine (2-CdA), were evaluated for response (Kim et al., Am J Hemato. 2009; 84:790-4). The corresponding overall response rates for those patients with indolent SM (N=22) were 60%, 0%, 14%, and 56%, respectively. Considering the entire evaluable study population (N=108), which included patients with more aggressive forms of mastocytosis such as aggressive SM, SM associated with another clonal hematological nonmast cell lineage disease (SM-AHNMD), and mast cell leukemia, the corresponding overall (and major) response rates were 53% (18%), 19% (0%), 18% (9%), and 55% (37%), respectively. Although the major response rates with these four cytoreductive agents were still suboptimal, the study concluded that 2-CdA and IFN-α constitute the treatments of choice, at the present time, for first line therapy in SM. It was noted however, that the degree and duration of response from these drugs remained inadequate and novel drugs are required to address this unmet need.
Interferon therapy has been used in mastocytosis because of its activity in myeloproliferative disorders. A few reports based on small series of patients have suggested that interferon therapy may induce some responses in the disease, even in some cases complete response. However, it has also been shown that interferon therapy cannot reduce mast cell infiltration in most cases. Furthermore, in mastocytosis interferon therapy is associated with a high rate of side effects and particularly with depression. As a consequence the dropout rate is very high and only few patients (>25%) can maintain therapy for a long period of time. A few cases suggest that corticosteroids and interferon together may improve response rate; however, corticosteroids are also associated with side effects. Thus, interferon with or without prednisone may be used in mastocytosis to reduce mast cell mediator release symptoms but its potential benefits must be weighed against its high rate of side effects.
Cladribine (Leustatin®) is a purine analogue that is efficient to induce apoptosis in resting cells. It has been used successfully in hairy cell leukemia and in Langerhans histiocytosis. Recent publications showed 2-CdA to effectively decrease symptoms associated with mediators release and also to reduce mast cell tumor burden in up to 50% of cases with few complete responses. However, relapses occur and maintenance therapy is probably needed in the majority of cases. Although well tolerated, 2-CdA administration induces an immunosuppressive state and although not yet fully demonstrated is potentially carcinogenic. Therefore, the feasibility of 2-CdA treatment in the long-term maintenance therapy of indolent mastocytosis is questionable.
The identification and prevalence of the D816V c-Kit tyrosine kinase mutation in mastocytosis has led to development of novel drugs directed against mast cells. Imatinib was the first of a new class of drugs known as small molecular weight tyrosine kinase inhibitors capable of blocking tyrosine kinase activity of c-Kit. In vitro experiments, however, showed that mast cells carrying the D816V c-Kit mutation were resistant to imatinib. Nevertheless, imatinib has been administered to mastocytosis patients with limited success in SM, although better response has been observed in rare cases of mastocytosis with transmembrane c-Kit mutations. Recently, a study by Vega Ruiz et al. (Leuk Res 2009; 33:1481-1484) showed that 6/11 indolent mastocytosis patients reported symptomatic improvements while receiving imatinib therapy, two of whom had the c-Kit D816V mutation. However, response was relatively short-lived, all patients developing resistance with reoccurrence of symptoms, leading to a conclusion that imatinib therapy did not result in appreciable clinical activity in patients with c-Kit D816V mutation. This unsatisfactory level of efficacy was confirmed in the Mayo Clinic retrospectively study (Kim et al., Am J Hemato. 2009; 84:790-4), with imatinib demonstrating a low overall response rate of 17% in c-Kit D816V positive SM patients, leading to the authors not endorsing the use of imatinib in patients with WHO-defined SM. Moreover, imatinib has shown cardiotoxicity related to its inhibition of the Abelson kinase (ABL), making its long-term use questionable for treatment of indolent forms of mastocytosis. In contrast to imatinib, a newer generation of tyrosine inhibitors dasatinib and midostaurin (PKC412) can inhibit the constitutive activity of the c-Kit D816V tyrosine kinase. However, when tested in vivo these drugs have also not lived-up to expectations, as seen in a phase 2 study that concluded dasatinib does not eliminate SM in the patients with c-Kit D816V mutation (Verstovsek et al., Clin Cancer Res. 2008; 14:3906-15).
There exists a continuing need to identify new targeted drugs that possess greater inhibitory action against c-Kit, with improved selectivity to minimize side effects, capable of inhibiting mast cell survival and release of mast cell mediators for treatment of mastocytosis with mast cell mediator release associated handicap, and in particular indolent forms of mastocytosis. In the absence of any single drug achieving a widespread response, it is possible that combination therapy based on different cytoreductive or disease modifying drugs may also be a viable strategy for both indolent forms and aggressive forms of mastocytosis.